My dad always said if a thief wants something, he's gonna do whatever he can to get it. If not a slashed top then a broken window. My 88 camaro got broken into and they tried to steal it. broke the steering column and the ignition was on when I got outside. what saved me was a kill switch I installed. so for my efforts, they stole the stereo. I had it locked and they got in thru the rear glass.
I've had 4 cars broken into and each time they took something. the first 3 they broke windows. the last one, I had it unlocked. However, the theifs left behind one of those emergency glass breakers so if it would have been locked, not only would my speaker box be gone but I would have had to pay for a broken window too.
I never lock my car any more.